Control properties of single motor units

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A motor unit is composed of a motor neuron in the spinal cord, it's axon that travels out to the muscle, and all the muscle fibres it innervates. Motor units provide the only output from the central nervous system to skeletal muscle and thus play a key role in the neural control of movement. Motor units are controlled by descending voluntary commands from the brain and feedback from sensory receptors. Impulses from sensory receptors activate motor units via reflex pathways through the spinal cord. A common way to study the reflex control of motor units is to deliver one or two pulses of electrical stimulation to a peripheral nerve and record the reflex response (H-reflex), mediated through the spinal cord, that returns to the muscle. This technique has provided a wealth of information about human reflexes. During human movement, however, motor units do not receive only one or two impulses along reflex pathways but rather they receive multiple impulses over several seconds. How motor units respond to such repetitive inputs has gone largely unexplored. In addition, motor units often remain active even when the stimulation is turned off, but the mechanisms that underlie these involuntary contractions remains controversial. The short term goals for the experiments described in this proposal are to characterize how motor units respond when they receive physiologically relevant trains of impulses along reflex pathways at different frequencies, intensities and at different levels of voluntary contraction. The long term goals are to identify the mechanisms responsible for: 1) the changes in reflex amplitude that occur during repetitive stimulation and 2) the involuntary muscle contractions that persist after the stimulation has been turned off. The results will identify how motor units respond when they receive inputs along reflex pathways that are more physiologically relevant than have been studied previously (i.e. trains of impulses rather than just one or two impulses). The findings may lead to new way of thinking about the neural control of human motor units.